This invention is directed to locating a plurality of offset points with reference to a peripheral surface or portions of a peripheral surface. Although the invention is to such locating it is believed easier to understand with reference to cutter bits for rotating drums of a coal mining machine for which the invention was initially developed. Accordingly, such bits and drums are described herein as the presently preferred embodiment of the invention.
In the mining of coal it is accepted practice to arrange cutter bits on a traveling or rotating member, such as a cutter chain or a rotating drum, such that the cutting edges or tips of the cutter bits travel through separate paths in the coal seam to be mined. There are various factors regarding the cutting of coal seams which are evaluated by various manufacturers of mining machinery in locating cutter bits on a rotating member. The locating of the cutter edges or cutting points of a cutter bit is referred to as the "lacing" of a cutter chain or drum and varies among various machinery manufacturers; however, all machinery manufacturers want as accurate locating of the cutter edge or point as is possible under the various manufacturing process. As is also known cutter bits have various forms of cutting edges with the conical or pointed bit being the preferred form of bit used by the Joy Technologies Inc. for which this invention was developed. It is to be realized that the principles of this invention are equally applicable to a linear edged bit since the center of the linear length is equivalent to the tip or pointed end of a conical bit. Accordingly, further description of this invention is with reference to a conical bit having an outer cutter bit point. Also a bit point is a point which is an outermost point of a bit.
Cutter drums vary in design for various mining machines including machines manufactured by a specific manufacturer. As is known, a cutting drum may consist of elongated drum sections, end sections and ring sections between the drum and end sections. The drum, end and ring segments form a cutter head with various cutter head designs being utilized. Regardless of the design of a cutter head or cutting chain it is necessary that the cutter head or chain cut its own clearance. That is, the bits on the cutter head cut and break the coal, rock or earth such that the cutter head can be moved forward into a coal seam. In this regard it is to be noted that coal is a frangible material and that the path of movement of a cutter bit through a coal seam to cause coal breakage is an important aspect of proper lacing of the cutting bits on a drum or cutter chain. Also, each section of a cutter head must carry cutter bits to cut its own clearance. A cutter head which cannot cut clearance for itself is not an acceptable mining machine.
Inasmuch as the principles of this invention in locating bit points (i.e., the points of the bits) applied to cutting chains and the sections of a cutter head the prior methods and apparatus for locating cutter bits on a cutter head are discussed for the sake of simplicity in understanding the invention. It is not believed necessary to discuss each aspect of the prior methods and apparatus since the more significant aspects of the prior art are discussed herein. It is also to be borne in mind that diameters and lengths of a cutter drum section vary so that while the procedures discussed are the same the tools and aids utilized will vary to compensate for the variations in drum diameter. It is also to be noted that a drum section carries various bit blocks at various locations which receive the bits, and pedestals upon which the bit blocks are mounted. The geometry of the bit blocks and bits is known and, for a specific combination, fixed.
In the past, it has been accepted practice to design each section of a cutting head to carry cutter blocks which specified the location of each block being specified. Once such drawings were received by the manufacturer the bit blocks were secured to the drum section by suitable welding processes such as arc welding. It is not known what processes other manufactures may employ; however, in the case of Joy Technologies, hand locating of the bit blocks on the drum section was utilized. Various gages were used to locate the bit blocks. It is to be realized that the bit blocks are formed from castings or forging with a specific lower surface which lower surface is secured to the drum section. Although a bit block is preferably formed with a concave lower surface there is no one concave surface which will provide a good fit with all diameters of drum sections or the various angles to the rotation axis of the drum segment at which the bit blocks entered. Consequently it was necessary in the past to essentially hand fit the various bit blocks to the drum segments by cutting off sections of the lower portion of the bit blocks. Torch cutting of bit blocks based upon estimates of the desired curvature of the lower surface of the bit block are quite unsatisfactory. In many instances filler materials were utilized to hold portions of the bit blocks relative to the drum surface prior to tack welding. Consequently, the tolerance on the bit point was quite large to compensate for the difficulties in locating the bit blocks and, in turn, the bit points.